I haven’t read everything, yet, but I don’t see anything that supports that the shift would have been abrupt. This is my point. There was a shift and it was gradual, as I would expect it to be. Voting Democrat was considered both Southern AND a matter of family (I am actually related to Jimmy Carter, by marriage. Not that he knows who I am or anything). A lot of my family voted for Carter. And he was also the last Democrat they voted for.
So I don’t see any support that any Republican strategy would have abruptly shifted the South to them. It WAS gradual. And it was as much by Republicans making themselves both religiously and socially identifiable as Southern.
There are still a few Southern Democrats that say “As long as they are Democrat and breathing, I’ll vote for them.” But their numbers are dwindling.